[LeetCode] Restore IP Addresses

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Use 3 pointers to iterate each possible starting position for each part, then check if each part is valid.

NOTE: The definition of "valid IP address"!!

class Solution {public:    vector<string> restoreIpAddresses(string s) {        int len = s.length();        vector<string> result;        if (len > 12 || len < 4) return result;        for (int i = 1; i <= len - 3; i ++) {            string s1 = s.substr(0,i);            if(!isValid(s1)) continue;            for (int j = i + 1; j <= len - 2; j ++) {                string s2 = s.substr(i,j-i);                if(!isValid(s2)) continue;                for (int k = j + 1; k <= len - 1; k ++) {                    string s3 = s.substr(j,k-j);                    string s4 = s.substr(k,len-k);                    if(!isValid(s3) || !isValid(s4)) continue;                    string tmp=s1+"."+s2+"."+s3+"."+s4;                    result.push_back(tmp);                }            }        }        return result;    }    bool isValid(string s) {    if (s.size() > 1 && s[0] == '0')        return false;    if (stoi(s) <= 255 && stoi(s) >= 0)        return true;    else        return false;    }};

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